Laser techniques have been developed to a great extent and it is possible at present to obtain various kinds or types of laser light by utilizing solid, gas, and liquid media. In many purposes of applications utilizing coherent laser light, laser light having various wavelengths is required and in some cases, a laser light exhibiting a continuous spectrum over a certain range of wavelengths is required. However, it is difficult to satisfy the aforesaid purposes or requirements even with the various kinds of laser light obtainable at present. Hence, a means has frequently been employed in which coherent laser light is converted into laser light of the second harmonic; that is to say, laser light of a wavelength of which the frequency is obtained is twice the fundamental frequency, by passing the laser light through a nonlinear optical crystal. Conversion of the wavelength of a laser light by a parametric effect using such a nonlinear optical crystal has also been investigated and developed.
In the prior art, monocrystalline forms of potassium dihydrogen phosphate (KDP), ammonium dihydrogen phosphate (ADP), barium sodium niobate (BaNaNbO.sub.3), and lithium niobate (LiNbO.sub.3) were used as coherent light phase modulators and for generating higher frequency harmonics. Monocrystalline KDP and ADP, while offering greater resistance to optical irradiation induced surface damage due to laser beam bombardment, do not exhibit large optical nonlinearities. This rendered these crystals unsuitable for higher harmonic frequency generation or conversion. In contrast, BaNaNbO.sub.3 and LiNbO.sub.3 showed large nonlinearities but, unfortunately, a low resistance to optical damage. In this regard, the term "resistance to optical damage" means the number of times the surface of a crystalline material can be bombarded (shots) with laser rediation of a given power density in watts per unit area before the subject crystal shows signs of opacity. Thus, a crystal showing high resistance would require a larger number of shots than a crystal of low resistance for the same power density of the incident laser beams.
All of the above-mentioned crystals vary their refractive index when an electric field is applied to them. Thus, such crystals find employment as electro-optic phase modulators. While these crystals can be used both in the form of powdered samples and large single crystals, it has been recognized that large single crystals are preferred because large electro-optical and nonlinear effects can be obtained through their use.
The possibility of using organic molecules in nonlinear optical devices has generated much interest recently because a large number of molecules are available for investigation. Some substituted aromatic molecules are known to exhibit large optical nonlinearities. The possibility of such an aromatic molecule having large optical nonlinearities is enhanced if the molecule has donor and acceptor groups bonded at opposite ends of the conjugated system of the molecule.
U.S. Pat. No. 4,199,698 discloses that the nonlinear optical properties of 2-methyl-4-nitroaniline (MNA) make it a highly useful material in nonlinear devices that convert coherent optical radiation including a first frequency into coherent optical radiation including a second frequency. The nonlinear devices have means for introducing coherent radiation of a first frequency into the MNA and means for utilizing coherent radiation emitted from the MNA at a second frequency.
U.S. Pat. No. 4,431,263 discloses that diacetylenes and polymers formed from diacetylenic species, which are amendable to close geometric, steric, structural, and electronic control, provide nonlinear optic, waveguide, piezoelectric, and pyroelectric materials and devices. Diacetylenes which are crystallizable into crystals having a noncentrosymmetric unit cell may form single crystals or be elaborated into a thin film upon a substrate by the Langmuir-Blodgett technique. Such films may be polymerized either thermally or by irradiation for use in nonlinear optical and other systems. Diacetylenes are covalently bonded to substrates through the employment of silane species and subsequently polymerized to yield nonlinear optic and other devices having high structural integrity in addition to high efficiencies and optical effects.
Sov. Phys. Crystallogr. 22, 305 (1977) discloses that a compound having the structure ##STR1## exhibits second harmonic generation. However, this paper does not suggest or predict that Schiff base compounds in general will exhibit second harmonic generation. Isomers of the aforementioned compound, e.g. ##STR2## pack in a centrosymmetric space group, and, consequently, observation of second harmonic generation is eliminated.